Vitamin D protects against stress fracture in adolescent girls

Vitamin D intake can cut the risk of stress fractures in adolescent girls in half, especially in those who engage in increased levels of high-impact activity, according to new study results.

Researchers evaluated the relationship between intake of vitamin D, calcium, and dairy products and stress fractures in 6,712 girls participating in the ongoing prospective cohort Growing Up Today Study. Participants were aged 9 to 15 years at baseline. Dietary intake was assessed every 12 to 24 months using a standardized questionnaire.

At baseline, the mean intake of calcium and vitamin D were below the recommended dietary allowance (RDA) for girls their age. During 7 years’ follow-up, 3.9% of the girls experienced a stress fracture; 90% of these injuries occurred in girls who were participating in at least 1 hour of high-impact activity per day. Girls in the highest quintile of vitamin D intake, from either diet or supplements, had a 50% lower risk of stress fracture than girls in the lowest quintile. Among girls who participated in at least 1 hour of high-impact activity per day, those in the highest quintile of vitamin D intake had a 52% lower risk of stress fracture compared with those in the lowest quintile.

In contrast, neither calcium nor dairy intake was related to risk of developing a stress fracture. In fact, contrary to expectations, high calcium intake was associated with an increased risk of stress fracture, a finding the researchers say warrants further investigation.

Researchers note that the results of their study support the recent increase in the RDA for vitamin D for adolescents from 400 IU to 600 IU per day, but it is not known whether higher doses would further decrease the risk of stress fracture.